**Abstract**

A novel experimental approach based on a toroid Cavendish balance is used to evaluate the product of photon mass squared and the ambient cosmic magnetic vector potential A. The method is based on the energy density of the vector potential in the presence of photon mass, not on measurement of magnetic field. The experiment discloses A m_{g}^{2} less than 2 x10^{-9} Tm/m^{2}, with m_{g}^{-1} as the characteristic length associated with photon mass. Consequently if the ambient magnetic vector potential is A approximately 10^{12} Tm due to cluster level fields, m_{g}^{-1} is greater than 2 x 10^{10} m. If we conservatively use galactic fields prior to a reversal, then m_{g}^{-1} is greater than 1 x 10^{9} m, a figure still superior to that derived from the Jovian magnetic field.

The photon mass is ordinarily assumed to be exactly zero. If there is any deviation from zero, it must be very small, since Maxwellian electromagnetism has been very well verified (in the classical domain). A nonzero photon mass would give rise to a wavelength dependence of the speed of light in free space, the possibility of longitudinal electromagnetic waves, a leakage of static electric signals into conductive enclosures, and a more rapid (exponential or Yukawa) falloff of magnetic dipole fields with distance than the usual inverse cube dependence.

Improvements in sensitivity of the present method can be achieved by conducting the experiments in a quieter location, use of a larger toroid, and by improving estimates of the ambient vector potential based on better mapping of cosmic magnetic fields.

**References and links**

[1] R. S. Lakes, "Experimental limits on the photon mass and cosmic magnetic vector potential", *Physical Review Letters *, **1998**, *80*, 1826-1829.

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Blurb: PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 361, March 4, 1998 by Phillip F. Schewe and Ben Stein.

Blurb from Science Now on a later, more sensitive experiment. "New Experimental Limit on the Photon Rest Mass with a Rotating Torsion Balance",
Jun Luo, Liang-Cheng Tu, Zhong-Kun Hu, and En-Jie Luan, Physical Review Letters, 081801, 26 February 2003, was at http://sciencenow.sciencemag.org/cgi/content/full/2003/304/1, moved by the journal.

Abstract: A "magnetic" photon hypothesis associated with magnetic monopoles is tested experimentally. These photons are predicted to easily penetrate metal. Experimentally the optical transmittance T of a metal foil was less than 2 x 10